CALCIUM, MAGNESIUM, ETC. 



135 



manufacture of glass and in the refining of some of the 

 metals where it is employed as a flux. 



168. Calcium Oxid. When calcium carbonate is sub- 

 jected to heat, as in lime kilns or specially constructed 

 furnaces, the carbon dioxid is separated and the oxid ob- 

 tained, payers of limestone and wood are placed alter- 

 nately in the lime kiln, as shown in Fig. 56; the combus- 

 tion of the wood furnishes the necessary heat for the de- 

 composition of the carbonate. Calcium oxid or quick- 

 lime readily combines with both the carbon dioxid and 

 moisture of the air, forming air-slaked lime. During this 

 process of slaking, there is a material increase in volume 

 often resulting in the bursting of the barrels in which the 

 lime is stored. Calcium oxid is used for the preparation 

 of calcium hydroxid and mortar. 



169. Calcium Hydroxid. When water is added to cal- 

 cium oxid or quicklime, the material 



undergoes the slaking process and cal- 

 cium hydroxid, Ca(OH) 2 , is produced. 



CaO + H 2 O = Ca(OH) 2 . 

 Calcium hydroxid or slaked lime 

 readily absorbs carbon dioxid from 

 the air and forms calcium carbonate. 

 Ca(OH) 2 is somewhat soluble in 

 water, forming what is commonly 

 called lime water. When carbon 

 dioxid is passed into lime water, the 

 solution becomes turbid, due to the Fig. 57. 



formation of CaCO,. This reaction furnishes a means 



